Environmental controls on coccolithophore calcification

Coccolithophores are major contributors to global marine planktonic calcification, and in nature coccolithophores are invariably calcified through almost all of their life cycle. The response of calcification to environmental factors is essential in understanding the persistence of coccolithophores through at least 220 million years of changing global environments, and their prospects for current environmental change. So far the responses examined have been at the level of acclimation rather than adaptation in evolution. Variation in results of CO2 manipulation experiments can be tentatively attributed to variation among genotypes rather than differences in experimental procedure. Comparisons of methods using the same genotype, and of several genotypes using a single method, suggest significant variation among genotypes. The general response is a decreased particulate inorganic carbon (PIC) to particulate organic carbon (POC) ratio in higher than present CO2 concentrations and vice versa for lower CO2 concentrations. Fewer studies have investigated the effect of other environmental factors. Decreased availability of phosphorus and, to a lesser extent, nitrogen, as well as decreasing photosynthetically active radiation (PAR) down to a certain low value increase PIC:POC, while variable results have been found for changes in ultraviolet radiation (UVR). Many of these results can be accommodated by considering the restriction of calcification to the G1 phase of the cell cycle and the length of this phase under different growth conditions. Fewer studies have investigated the interactions among environmental factors which change with increased CO2 and increasing sea surface temperature; the shoaling of the thermocline will increase the mean PAR and UVR whilst decreasing nitrogen and phosphorus availability. More studies of these interactions, as well as of genetic adaptation in response to changed environmental factors, are needed.